Decoding method and apparatus enabling fast channel change of compressed video

ABSTRACT

A video decoder, and corresponding method are described for enabling fast channel change of compressed video, where a video decoder for receiving compressed stream data and providing decompressed video output includes receiving the compressed stream data and separating the normal stream and the channel change stream, a normal decoding portion for selectably receiving the compressed normal and channel change streams, and providing decompressed video output, and a normal frame store in signal communication with the normal decoding portion for storing reference pictures.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/478,923 (Attorney Docket No. PUO30170), filed Jun. 16, 2003 andentitled “METHOD AND APPARATUS ENABLING FAST CHANNEL CHANGE OFCOMPRESSED VIDEO”, which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention is directed towards video encoders and decoders(CODECs), and more particularly, towards an apparatus and method forreducing the perceived delay for the initial display of decoded videocontent following a channel change.

BACKGROUND OF THE INVENTION

Popular video compression standards, such as MPEG-2 and JVT/H.264/MPEGAVC, use intra and inter coding. For proper decoding, a decoder decodesa compressed video sequence beginning with an intra-coded (I) picture,and then continues to decode the subsequent inter-coded (P and B)pictures. A Group of Pictures (GOP) may include an I picture and severalsubsequent P and B pictures. I pictures typically require many more bitsto code than does a P or B picture of equivalent video quality.

When a receiver initially begins receiving a program on a particularchannel, such as following a channel change or initial turning on of thereceiver, it must wait until an I picture is received to begin decodingproperly, which causes a delay. To minimize channel change delay indigital video broadcast systems, I pictures are typically sentfrequently, such as every N pictures. For example, to enable ½ seconddelay of the video decompression portion of the system, it is common touse N=15 for 30 fps content. Because compressed I pictures are so muchlarger than compressed P and B pictures, this considerably increases thebitrate over what would be required if I pictures were not inserted sofrequently.

Most broadcast systems transmit I pictures frequently, for example every½ second, in order to limit the channel change delay time due to thevideo decoding system. In some systems, instead of sending full Ipictures frequently, a technique called “progressive refresh” is used,where sections of pictures are intra coded. Typically, all macroblocksin the picture are intra-coded at least once during an N-picture period.

In the JVT/H.264/MPEG AVC compression standard, P and B pictures may bepredicted using multiple reference pictures, including the picturesbefore a preceding I picture. The standard identifies random accesspoints as Independent Decoder Refreshes, or IDRs, which constrain thatno reference pictures before each IDR are used in predicting picturesfollowing the IDR.

The JVT/H.264/MPEG AVC compression standard includes a tool calledredundant pictures, defined in the standard as:

-   -   redundant coded picture: A coded representation of a picture or        a part of a picture. The content of a redundant coded picture        shall not be used by the decoding process for a bitstream        conforming to this Recommendation | International Standard. A        redundant coded picture is not required to contain all        macroblocks in the primary coded picture. Redundant coded        pictures have no normative effect on the decoding process. See        also primary coded picture.    -   The slice header contains a redundant_pic_cnt field, whose        semantics are defined as:    -   redundant_pic_cnt shall be equal to 0 for slices and slice data        partitions belonging to the primary coded picture. The        redundant_pic_cnt shall be greater than 0 for coded slices and        coded slice data partitions in redundant coded pictures. When        redundant_pic_cnt is not present, its value shall be inferred to        be equal to 0. The value of redundant_pic_cnt shall be in the        range of 0 to 127, inclusive.        -   If the syntax elements of a slice data partition A RBSP            indicate the presence of any syntax elements of category 3            in the slice data for a slice, a slice data partition B RBSP            shall be present having the same value of slice_id and            redundant_(—pic)_cnt as in the slice data partition A RBSP.        -   Otherwise (the syntax elements of a slice data partition A            RBSP do not indicate the presence of any syntax elements of            category 3 in the slice data for a slice), no slice data            partition B RBSP shall be present having the same value of            slice_id and redundant_pic_cnt as in the slice data            partition A RBSP.

Accordingly, what is needed is an apparatus and method for reducing theperceived delay for the initial display of decoded video contentfollowing a channel change.

SUMMARY OF THE INVENTION

These and other drawbacks and disadvantages of the prior art areaddressed by an apparatus and method that provide for fast channelchange of compressed video content.

A video decoder for receiving compressed stream data and providingdecompressed video output includes a demultiplexor for receiving thecompressed stream data and separating the normal stream and the channelchange stream, a normal decoding portion in signal communication withthe demultiplexor for receiving the compressed normal and channel changestreams, and providing decompressed video output, and a normal framestore in signal communication with the normal decoding portion forstoring reference pictures.

These and other aspects, features and advantages of the presentinvention will become apparent from the following description ofexemplary embodiments, which is to be read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood in accordance with thefollowing exemplary figures, in which:

FIG. 1 shows a block diagram for an encoder using normal resolution forthe channel change stream in accordance with principles of the presentinvention;

FIG. 2 shows a block diagram for a decoder using normal resolution forthe channel change stream in accordance with principles of the presentinvention;

FIG. 3 shows a block diagram for an encoder using a low-pass filteredchannel change stream in accordance with principles of the presentinvention;

FIG. 4 shows a table for an exemplary picture pattern in accordance withprinciples of the present invention;

FIG. 5 shows a block diagram for an encoder using a downsampled channelchange stream in accordance with principles of the present invention;

FIG. 6 shows a block diagram for a decoder using a downsampled channelchange stream in accordance with principles of the present invention;

FIG. 7 shows a block diagram for a decoder using a downsampled channelchange stream and a postprocessing filter in accordance with principlesof the present invention;

FIG. 8 is a flow chart of a video encoding method in accordance withprinciples of the present invention; and

FIG. 9 is a flow chart of a video decoding method in accordance withprinciples of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention provide allow channel change delayat any desired rate with a lower bitrate than prior art methods. Thatis, the invention enables low delay channel change time in a compressedvideo broadcast system, while significantly reducing the bitrate overprior methods of enabling low-delay channel change. In theJVT/H.264/MPEG AVC standard, individual P and B pictures are coded usingone or more different slice types (I, P and/or B), while I pictures arecoded using only I slices. Accordingly, in the description that follows,the term “slice” may be substituted for the term “picture” depending onthe context and applicable standard. Prior art systems broadcast Ipictures frequently to enable channel change, for example every Npictures. In embodiments of the present invention, normal I pictures aresent less frequently, and additional lower quality I pictures are sentmore frequently.

In accordance with the principles of the present invention, a desiredchannel change delay can be achieved without requiring I pictures to besent as frequently as is done in prior art systems. Instead, additionallower quality coded pictures, included in what is herein called thechannel change stream, are sent in addition to the normal quality codedpictures. In the channel change stream, lower quality I pictures aresent at the desired channel change frequency, and are used at thedecoder during the initial period following a channel change. Normalquality I pictures are sent in the normal stream at a lower frequency,and are used at the decoder once they are available.

For example, consider a system that sends I pictures in the normalstream every N*K pictures and lower quality I pictures in the channelchange stream every N pictures, with K>1. Each coded picture in thechannel change stream corresponds to a normal stream coded picture.Thus, when a coded picture is present in the channel change stream, twocoded representations of that picture are actually transmitted.

When a channel change occurs, a decoding system starts decoding thecompressed video as soon as it receives an I picture, either from thenormal stream or from the channel change stream. If the first I pictureto arrive is from the normal stream, the decoder continues normally.However, if the first I picture to arrive is a lower quality I picturefrom the channel change stream, the decoder decodes and uses the lowerquality I picture. This causes lower quality video to be displayed untila normal quality I picture arrives. This period of lower quality videois not significantly noticeable to a viewer as it is of short durationand immediately follows a channel change. The human visual system takessome time to adjust to a new visual scene.

The channel change stream may either contain only lower quality Ipictures, or may contain lower quality I, P and B pictures. The picturerate of the channel change stream may be lower than that of the normalstream. The lower quality pictures may be of the same resolution as thenormal pictures but encoded at a lower bitrate, or may be of a lowerresolution than the normal pictures. The bitstream size of the lowerquality I coded pictures in the channel change stream are small comparedwith the size of normal quality coded I pictures in the normal stream.So even though additional coded representations of the same picture arebeing transmitted, overall bitrate savings occur because the size of anormal quality P or B picture plus the lower quality I picture istypically significantly less than that of a normal quality I picturealone.

If the channel change stream contains low quality I, P and B pictures,after a channel change the decoder system waits for the arrival of alower quality I picture, and then it decodes and displays the lowerquality pictures from the channel change stream until a normal quality Ipicture is received, at which point it switches to the normal qualitystream.

If the channel change stream contains only I pictures, the decodingsystem waits for the arrival of an I picture in either the normal streamor channel change stream after a channel change. If the first I pictureto arrive is in the channel change stream, the decoding system decodesand displays the lower quality I picture. Then this lower qualitypicture is stored in the normal decoder picture stores and the decodingsystem begins decoding the subsequent normal stream P and B pictures,using the lower quality I picture from the channel change stream as areference. Because these normal quality P and B pictures wereinter-coded based on prior pictures in the normal stream rather than thecorresponding lower quality I picture from the channel change stream,this will cause some decoding drift.

Experiments have shown, however, that the visual impact of such drift issmall in this situation, because it lasts for only a short duration andimmediately follows a scene change corresponding to the channel change.The encoder can manage how much drift would occur and adjust codingparameters of the normal and/or channel change stream picturesappropriately such that drift does not exceed reasonable limits.

The instant description illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements that, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the block diagrams presented herein represent conceptual views ofillustrative circuitry embodying the principles of the invention.Similarly, it will be appreciated that any flow charts, flow diagrams,state transition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedia and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

The functions of the various elements shown in the figures may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When provided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (“DSP”)hardware, read-only memory (“ROM”) for storing software, random accessmemory (“RAM”), and non-volatile storage.

Other hardware, conventional and/or custom, may also be included.Similarly, any switches shown in the figures are conceptual only. Theirfunction may be carried out through the operation of program logic,through dedicated logic, through the interaction of program control anddedicated logic, or even manually, the particular technique beingselectable by the implementer as more specifically understood from thecontext.

In the claims hereof, any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementsthat performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means that can provide those functionalities as equivalentto those shown herein.

As shown in FIG. 1, an encoder in accordance with principles of thepresent invention is indicated generally by the reference numeral 100.The encoder 100 uses the same resolution for each of the normal streamand the channel change stream, and includes a normal encoder portion 130for producing the normal stream and a low-quality encoder portion 140for producing the channel change stream, each receiving the sameresolution input pictures. The normal portion 130 and the low-qualityportion 140 are each coupled in signal communication with a multiplexor(mux) 150, for providing the normal and channel change streams,respectively, to the mux 150 for transmission.

Turning to FIG. 2, a decoder in accordance with principles of thepresent invention is indicated generally by the reference numeral 200.The decoder 200 uses the same resolution for each of the normal streamand the channel change stream, and includes a demultiplexor (demux) 210for receiving compressed video data and demultiplexing the normal andchannel change streams, each coupled in selectable signal communicationwith a normal decoder portion 212. The normal decoder portion 212 iscoupled in signal communication with frame stores 214, and outputsdecoded video to a display.

Thus, FIG. 1 shows an encoding system and FIG. 2 shows a decoding systemthat each use the same resolution images for both the normal stream andthe channel change stream. In operation, the normal encoder portioncreates normal quality compressed video pictures for the normal stream,and a parallel low-quality encoder portion creates lower qualitycompressed video pictures for the channel change stream. The encoder 100shows two separate encoder blocks for the two encoder functions, but, aswill be recognized by those of ordinary skill in the pertinent art, thetwo encoder functions could be performed using the same encoder device.The normal stream and channel change stream are multiplexed together, ifnecessary, and transmitted. In the decoding system, a demux separatesthe normal stream and channel change stream, and a selection is made asto whether the picture from the normal stream or the channel changestream should be sent to the decoder.

Turning now to FIG. 3, an encoder in accordance with principles of thepresent invention is indicated generally by the reference numeral 300.The encoder 300 uses filtered data for the channel change stream, andincludes a normal encoder portion 330 for producing the normal stream,and a low-pass filter 332 in signal communication with a low-qualityencoder portion 340 for producing the filtered channel change stream.The normal portion 330 and the low-quality portion 340 are each coupledin signal communication with a mux 350, for providing the normal andchannel change streams, respectively, to the mux 350 for output.

Thus, FIG. 3 shows an alternate encoding system, which applies alow-pass filter to the input pictures prior to the lower qualityencoder. Because the pictures in the channel change stream are coded ata relatively low bitrate, they may contain visible coding artifacts. Bylow-pass filtering the images prior to encoding, some of these visiblecoding artifacts may be removed.

The multiplexor arranges the transmission time of the coded picturessuch that the channel change stream I pictures are interspersed with thenormal stream coded pictures. The channel change stream coded picture ispreferably transmitted near the time that the normal stream picturecorresponding to the same input picture is transmitted, and before anynormal stream pictures that are inter-predicted with respect to thatpicture.

As shown in FIG. 4, an exemplary picture pattern is indicated generallyby the reference numeral 400 for the case where only I pictures areincluded in the channel change stream, with N=12 and K=3. For anexemplary 24 fps sequence where channel change start periods of ½ secondare desired, lower quality I pictures are inserted in the channel changestream every 12 pictures. Normal quality I pictures are inserted in thenormal stream every 36 pictures.

Consider the case where a receiver tuned in to the channel while picture5 was being received. The receiver would then wait until the first Ipicture in either stream arrived, which in this example is 12 in thechannel change stream, and decode and display it. The decoded lowerquality I picture 12 from the channel change stream would then be placedin the decoder's picture store, and used in decoding pictures 12-23 fromthe normal stream. These decoded pictures will contain drift. Whennormal quality P picture 24 arrives in the normal stream, the receivermay either choose to decode the normal stream's P picture 24 or thechannel change stream's I picture 24. This could either be a receiverend decision, or a preference could be signaled by the encoder in thebitstream, based on which one will yield less drift. The normal stream'spictures 25-35 are then decoded, still with drift. Once the normalstream's picture 36 is received, which is an I picture, the decoder canstart decoding properly without added drift for all subsequent pictures.From the viewer's perspective, for a short period after a channelchange, here up to about 1.5 seconds, lower quality video is displayed,and then normal quality is displayed.

Bitrate savings versus a prior art system is achieved because the largenormal stream I pictures are sent less frequently that they would besent in a prior art system. The lower quality I pictures sent in thechannel change stream are much smaller than the normal quality Ipictures. An encoding system may send the lower quality I pictures inthe channel change stream as frequently as desired, and with anypattern. I pictures in the normal stream also need not follow a regularpattern, and for example may be inserted whenever a scene change occurs.An encoding system does not need to insert channel change stream Ipictures if the distance between I pictures in the normal stream doesnot exceed a desired value. The encoding system may choose to insert Ipictures in the channel change stream whenever necessary to maintain amaximum I picture spacing, associated with a desired channel changedelay limit, for example.

The channel change stream may contain pictures of different resolutions.For example, some of the I pictures in the channel change stream may beof the same resolution as the normal stream and others may be at a lowerresolution. Alternately, two or more different lower resolutions forpictures in the channel change stream may be used.

In order to reduce the drift that occurs when decoding a normal streampicture using a channel change stream picture as a predictor, theencoder can restrict the range of allowable reference pictures for the Pand B pictures that follow the normal stream picture that corresponds tothe channel change stream I picture. In the JVT/H.264 video compressionstandard, P and B pictures may be predicted using multiple referencepictures, which provides a coding efficiency advantage over using asingle reference picture. For the example in FIG. 4, a restriction maybe imposed such that pictures 12-23 in the normal stream may not usereference pictures prior to picture 12. If this restriction were notimposed, more drift would occur following a channel change. For example,if picture 15 was predicted from both pictures 12 and 9, and a channelchange occurred while picture 5 was being received, the decoding systemwould have a representation of picture 12, from the channel changestream I picture 12, but would not have any representation of picture 9.This could lead to a significant reduction in visual quality whendecoding pictures 13-23. However, if a restriction were imposed thatpicture 15 be predicted only from picture 12, this significant drift canbe avoided, with a small penalty in the coding efficiency of picture 15.

Turning to FIG. 5, an encoder in accordance with principles of thepresent invention is indicated generally by the reference numeral 500.The encoder 500 uses downsampled data for the channel change stream, andincludes a normal encoder portion 530 for producing the normal stream,and a downsampler 534 in signal communication with a low-quality encoderportion 540 for producing the filtered channel change stream. The normalportion 530 and the low-quality portion 540 are each coupled in signalcommunication with a mux 550, for providing the normal and channelchange streams, respectively, to the mux 550 for output.

Turning now to FIG. 6, a decoder in accordance with principles of thepresent invention is indicated generally by the reference numeral 600.The decoder 600 uses downsampled data for the channel change stream, andincludes a demux 610 for receiving compressed video data, coupled insignal communication with each of a normal decoder portion 612 and alower resolution decoder portion 618. The normal decoder portion 612 iscoupled in signal communication with frame stores 614, and selectablyoutputs decoded video to a display and to the frame stores 614. Thelower resolution decoder portion 618 is coupled in signal communicationwith frame stores 620, and outputs decoded video to an upsampler 622,which, in turn, selectably outputs upsampled decoded video to a displayand to the frame stores 614.

Thus, FIG. 5 shows an encoding system and FIG. 6 shows a decodingsystem, each of which use lower resolution images for lower qualitypictures in the channel change stream than for the normal pictures. Forexample, 704×480 pixels could be used for the normal pictures and352×240 pixels for the channel change pictures. The input pictures areencoded normally for the normal stream, and are resized to a lowerresolution and encoded at that lower resolution for the channel changestream. The normal stream and the channel change stream are multiplexedtogether and transmitted. In the decoding system, a demux separates thenormal stream and the channel change stream, and a selection is made asto whether the picture from the normal stream or the channel changestream should be decoded and displayed. If the channel change stream isdecoded and displayed, the decoded picture is put into the normal streamdecoder's picture store for use in decoding subsequent normal streamcoded pictures. Although separate blocks are provided in the figure forthe normal decoder and lower quality decoder, both functions may beperformed using a single device as will be recognized by those ofordinary skill in the pertinent art.

In the decoding system, following a channel change, lower quality videois initially displayed, and once an I picture in the normal stream isreceived, normal quality video begins to be displayed. The abrupttransition from lower quality video to normal quality video may be morenoticeable to a viewer than the lower quality video itself. To reducethe abruptness of the transition, a postprocessor may be added followingthe decoder to filter the decoded pictures. The filter strength may beadjusted over several pictures, to gradually increase the resolution orquality of the decoded pictures.

Turning now to FIG. 7, a decoder in accordance with principles of thepresent invention is indicated generally by the reference numeral 700.The decoder 700 uses downsampled data for the channel change stream, andincludes a demux 710 for receiving compressed video data, coupled insignal communication with each of a normal decoder portion 712 and alower resolution decoder portion 718. The normal decoder portion 712 iscoupled in signal communication with frame stores 714, and selectablyoutputs decoded video to a display and to the frame stores 714. Thenormal decoder portion 712 is further coupled in signal communicationwith a post processing filter 716, which selectably outputspostprocessed decoded video to the display and to the frame stores 714.The lower resolution decoder portion 718 is coupled in signalcommunication with frame stores 720, and outputs decoded video to anupsampler 722, which, in turn, selectably outputs upsampled decodedvideo to a display and to the frame stores 714.

Thus, FIG. 7 shows a decoding system that incorporates a post processingfilter. A post processing function may also be added following the lowerquality decoder to hide compression artifacts.

As shown in FIG. 8, a video encoding method for receiving input picturesand providing compressed stream data is indicated generally by thereference numeral 800. The method 800 includes a start block 810 thatpasses control to an input block 812 for receiving input pictures. Theinput block 812 passes control to a function block 814 for encodingnormal stream data from the received input pictures. The function block814, in turn, passes control to a function block 816 for encodingchannel change stream data from the received input pictures, where thechannel change stream data includes lower-quality encoded data than thenormal stream data. The function block 816 passes control to a functionblock 818 for multiplexing the normal and channel change data streamsinto a combined output stream, and, in turn, passes control to an endblock 820.

Turning to FIG. 9, a video decoding method for receiving compressedstream data and providing decompressed video output is indicatedgenerally by the reference numeral 900. The method 900 includes a startblock 910 that passes control to an input block 912 for receiving thecompressed stream data, which passes control to a function block 914 forseparating the normal stream and the channel change stream. The functionblock 914 passes control to a function block 916 for receiving at leastone of the compressed normal and channel change streams, and providingdecompressed video output. The function block 916, in turn, passescontrol to a function block 918 for storing reference pictures for usein decoding inter-coded pictures. The function block 918 passes controlto an end block 920.

There are several possible ways in which the multiplexing of the normalsteam and channel change stream may be performed. To enable a backwardscompatible system where the normal stream can be decoded withoutalteration to pre-existing decoders, one method to is place the channelchange stream's lower quality coded pictures in user data associatedwith the corresponding picture of the normal stream.

This method allows the decoding system to identify the picture time of achannel change stream coded picture. If this method is used, analteration to the unique picture start code of the coded pictures in thechannel change stream is necessary, such as by using bit or bytestuffing, to help the pre-existing normal decoder avoid detecting thepicture start code of the channel change stream picture inside of theuser data. The bit or byte stuffing procedure may be reversed in thedecoding system, before passing data to a standards compliant decoder.

An alternative multiplexing method is to use a different PID for thechannel change stream than for the normal stream. In this case, thechannel change stream will need to include timing information for thecoded pictures, synchronized with the normal stream pictures. Also, anassociation must be made between the PIDs of the normal stream and thechannel change stream.

If the JVT/H.264/MPEG AVC compression standard is used in this system,and the resolution of the normal pictures and channel change picturesare identical, the redundant pictures syntax of JVT may be used forcoding the channel change pictures by setting the redundant_pic_cntfield in the slice header to 1 for the channel change pictures. In thiscase, in the decoding system, the channel change stream pictures may beidentified by searching for pictures containing a redundant_pic_cntfield equal to 1 in the slice header.

These and other features and advantages of the present invention may bereadily ascertained by one of ordinary skill in the pertinent art basedon the teachings herein. It is to be understood that the principles ofthe present invention may be implemented in various forms of hardware,software, firmware, special purpose processors, or combinations thereof.

Most preferably, the principles of the present invention are implementedas a combination of hardware and software. Moreover, the software ispreferably implemented as an application program tangibly embodied on aprogram storage unit. The application program may be uploaded to, andexecuted by, a machine comprising any suitable architecture. Preferably,the machine is implemented on a computer platform having hardware suchas one or more central processing units (“CPU”), a random access memory(“RAM”), and input/output (“I/O”) interfaces. The computer platform mayalso include an operating system and microinstruction code. The variousprocesses and functions described herein may be either part of themicroinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU. In addition,various other peripheral units may be connected to the computer platformsuch as an additional data storage unit and a printing unit.

It is to be further understood that, because some of the constituentsystem components and methods depicted in the accompanying drawings arepreferably implemented in software, the actual connections between thesystem components or the process function blocks may differ dependingupon the manner in which the present invention is programmed. Given theteachings herein, one of ordinary skill in the pertinent art will beable to contemplate these and similar implementations or configurationsof the present invention.

Although the illustrative embodiments have been described herein withreference to the accompanying drawings, it is to be understood that thepresent invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one ofordinary skill in the pertinent art without departing from the scope orspirit of the present invention. All such changes and modifications areintended to be included within the scope of the present invention as setforth in the appended claims.

What is claimed is:
 1. In a video decoder, a video decoding method thatreceives compressed stream data and provides decompressed video output,the method comprising: receiving the compressed stream data andseparating a normal stream and a channel change stream, the normalstream and the channel change stream each being generated external tothe video decoder and comprising a plurality of pictures for a sameprogram, wherein the channel change stream comprises images with lowerresolution than images of the normal stream; receiving at least one ofthe compressed normal and channel change streams, and providingdecompressed video output; and storing reference pictures for use indecoding inter-coded pictures.
 2. The video decoding method of claim 1further comprising: selecting a compressed picture to decode from one ofthe normal stream and the channel change stream; and upsampling thelower resolution channel change stream images.
 3. The video decodingmethod of claim 2 further comprising: decoding redundant picture syntaxin compliance with a video compression standard.
 4. The video decodingmethod of claim 2 further comprising: responding to a signal indicatingwhether to use normal stream or channel change stream pictures forsubsequent channel change stream intra-coded pictures.
 5. The videodecoding method of claim 2 further comprising: postprocessing the outputof the normal decoder to reduce the abruptness of a transition fromlower-quality to normal quality output.
 6. A video decoder comprising: areceiver that receives a compressed stream data and separating a normalstream and a channel change stream, the normal stream and the channelchange stream each being generated external to the video decoder andcomprising a plurality of pictures for a same program, wherein thechannel change stream comprises images with lower resolution than imagesof the normal stream; a decoder that receives at least one of thecompressed normal and channel change streams, and providing decompressedvideo output; and a storage that stores reference pictures for use indecoding inter-coded pictures.
 7. The video decoder of claim 6 furthercomprising: a selector that selects a compressed picture to decode fromone of the normal stream and the channel change stream; and an upsamplerthat upsampes the lower resolution channel change stream images.
 8. Thevideo decoder of claim 7, wherein the decoder decodes redundant picturesyntax in compliance with a video compression standard.
 9. The videodecoder of claim 7, wherein the decoder responds to a signal indicatingwhether to use normal stream or channel change stream pictures forsubsequent channel change stream intra-coded pictures.
 10. The videodecoder of claim 7 wherein the decoder postprocesses the output of thenormal decoder to reduce the abruptness of a transition fromlower-quality to normal quality output.